Incineration, an attractive alternative to burying in a landfill for the disposal of urban garbage, is practiced throughout the world and results in a considerable decrease in waste volume and the recovery of energy in the form of steam or electricity. One of the significant drawbacks to the incineration procedure is that several hundred stable and toxic compounds, including polychlorinated dibenzo-p-dioxins (collectively commonly termed "dioxins") and polychlorinated dibenzofurans (collectively commonly termed "furans"), are formed and are presented in parts-per-million concentrations both in the flyash formed during combustion and in the stack emissions.
A large city may incinerate 3 to 5 million tons of garbage annually. For every million tons of urban waste incinerated, about 34,000 tons of flyash are produced by the typical incinerator. Between 95 and 99% of the flyash is precipitated electrostatically and buried in landfills. The remainder is emitted from the incinerator stacks along with the gaseous by-products, namely water vapour, HCl, SO.sub.2, CO.sub.2, air and volatilized organic compounds. The gaseous stack emission introduces dioxins, furans and other toxic chlorinated compounds to the atmosphere. Landfill disposal of flyash introduces dioxins, furans and other hazardous organic chlorinated compounds into the earth from where they may be leached into ground water systems.
The primary hazard of the most toxic of these organic compounds, dioxins and furans, to humans may be cancer in the long term, but dioxins exert a much higher impact on the general environment and are considered undesirable. There exists, therefore, a need for a means to decrease the dioxins, furans and other chlorinated compounds content of both solid and gaseous by-products from incinerator systems. It is also essential to decrease emissions of acid gases produced in the incineration process.
There has been previously described in U.S. Pat. No. 4,793,270, naming two of us as inventors, the disclosure of which is incorporated herein by reference, the surprising discovery that the flyash which is formed during the incineration of solid municipal waste catalyses the formation of dioxins from chlorinated phenols formed from combustion products of plastics, paper and chemicals, and several other dioxin precursors in the gaseous combustion products. Accordingly, as described in that patent, a material acting as a catalyst inhibitor is provided in association with the flyash so as to inhibit the catalytic activity of the flyash towards the formation of chlorinated compounds including dioxins and furans.